Nonaqueous electrolytic solution and lithium secondary battery employing the same

ABSTRACT

1. A non-aqueous electrolytic solution comprising an electrolytic salt in a non-aqueous solvent, which further contains a pentafluorophenoxy compound having the formula (I): 
                         
in which R represents a substituting group such as an alkylcarbonyl group, an alkoxycarbonyl group, an aryloxy-carbonyl group, and an alkanesulfonyl group, under the condition that at least one hydrogen atom contained in the substituting group can be substituted with a halogen atom or an aryl group.

FIELD OF INVENTION

The present invention relates to a non-aqueous electrolytic solutionwhich imparts to lithium secondary batteries favorable batteryperformances in the electric capacity and the storage property andfurther in the battery cycle property, and further relates to a lithiumsecondary battery containing the same.

BACKGROUND OF INVENTION

At present, a lithium secondary battery is generally employed as anelectric source for driving small electronic devices. The lithiumsecondary battery essentially comprises a positive electrode, anon-aqueous electrolytic solution, and a negative electrode. A lithiumsecondary battery utilizing a positive electrode of lithium compoundoxide such as LiCoO₂ and a negative electrode of carbonaceous materialor lithium metal is preferably used. As the electrolytic solution forthe lithium secondary battery, a carbonate such as ethylene carbonate(EC) or propylene carbonate (PC) is preferably used.

Heretofore, there have been proposals to incorporate a variety ofadditives to a non-electrolytic solution so as to improve variousperformances of a lithium secondary battery.

U.S. Pat. No. 5,709,968 (corresponding to Japanese Patent ProvisionalPublication No. 9-50822) describes a number of compounds includingp-fluoroanisole and 2,4-difluoroanisole to be used as additives forkeeping a lithium secondary battery from over-charging.

Japanese Patent Provisional Publication No. 11-329490 describespentafluorobenzene derivatives in which the six hydrogen atoms on thebenzene ring are replaced with five fluorine atoms and one substitutedester group, substituted acyl group or trifluoromethyl group, such asmethyl pentafluorobenzenecarboxylate and octafluorotoluene, which areemployable for improving the cycle performance of a lithium secondarybattery.

DISCLOSURE OF INVENTION

Further improvements of various lithium battery performances aredesired. Particularly, the improvement of cycle performance (i.e., rateof retention of discharge capacity after repeated charging-dischargingprocedures) of a lithium secondary battery is highly desired.

In a lithium secondary battery using LiCoO₂, LiMn₂O₄ or LiNiO₂ as thepositive electrode material, a portion of a solvent of the non-aqueouselectrolytic solution oxidatively decomposes in the course of charging,and the decomposition product disturbs the desired electrochemicalreaction of the battery. Accordingly, the battery performances lower. Itis assumed that this phenomenon arises from electrochemical oxidation ofthe solvent in contact with the surface of the positive electrode.

On the other hand, in a lithium secondary battery using a highlycrystalline carbonaceous material such as natural graphite or artificialgraphite as the negative electrode material, the solvent in thenon-aqueous electrolytic solution reductively decomposes on the surfaceof the negative electrode in the course of charging. The reductivedecomposition occurs in a portion of the solvent in the repeatedcharging-discharging procedures even if the solvent contains EC(ethylene carbonate) which is generally employed as a solvent of thenon-aqueous electrolytic solution, and hence the battery performanceslower.

For these reasons, the lithium secondary batteries still are notsatisfactory in their battery performances such as cycle performance andelectric capacity.

The present invention has an object to provide a non-aqueouselectrolytic solution which solves the above-mentioned problemsconcerning lithium secondary batteries and which is effective to impartan improved performance in the electric capacity, stability in thecharged condition, and the cycle performance to a lithium secondarybattery. The invention further has an object to provide a lithiumsecondary battery employing the non-aqueous electrolytic solution.

The present invention resides in a non-aqueous electrolytic solutioncomprising an electrolytic salt in a non-aqueous solvent, which furthercontains a pentafluorophenoxy compound having the formula (I):

in which R represents a substituting group selected from the groupconsisting of an alkylcarbonyl group having 2-12 carbon atoms, analkoxycarbonyl group having 2-12 carbon atoms, an aryloxycarbonyl grouphaving 7-18 carbon atoms, and an alkanesulfonyl group having 1-12 carbonatoms, provided that at least one hydrogen atom contained in thesubstituting group can be replaced with a halogen atom or an aryl grouphaving 6-18 carbon atoms.

The invention further resides in a lithium secondary battery whichcomprises a positive electrode, a negative electrode, and a non-aqueouselectrolytic solution comprising an electrolytic salt in a non-aqueoussolvent, in which the non-aqueous electrolytic solution further containsa pentafluorophenoxy compound of the above-mentioned formula (I).

In the invention, R of the formula (I) is a substituting group selectedfrom the group consisting of an alkylcarbonyl group having 2-12 carbonatoms, an alkoxycarbonyl group having 2-12 carbon atoms, anaryloxycarbonyl group having 7-18 carbon atoms, and an alkanesulfonylgroup having 1-12 carbon atoms. At least one hydrogen atom contained inthe substituting group can be replaced with a halogen atom or an arylgroup having 6-18 carbon atoms.

The substituting groups for R are described herein-below in detail.

Examples of the alkylcarbonyl groups having 2-12 carbon atoms includemethylcarbonyl, ethylcarbonyl, propylcarbonyl, butylcarbonyl,pentylcarbonyl, hexylcarbonyl, heptylcarbonyl, octylcarbonyl,nonylcarbonyl, decylcarbonyl, and dodecylcarbonyl. Further, branchedalkylcarbonyl groups such as isopropylcarbonyl, tert-butylcarbonyl and2-ethylhexylcarbonyl can be mentioned. Furthermore, substituting groupsin which at least one hydrogen atom contained in the substituting groupis replaced with a halogen atom or an aryl group having 6-18 carbonatoms can be mentioned. Examples of the last substituting groups arealkylcarbonyl groups such as trifluoromethylcarbonyl,1,2-dichloroethylcarbonyl, pentafluoroethylcarbonyl,heptafluoropropylcarbonyl, and benzylcarbonyl. Further, an alkylcarbonylgroup in which an alkyl substituent having an unsaturated bonding suchas methylene (CH₂═) or allyl (CH₂═CH—CH₂—) can be mentioned. Examplesare vinylcarbonyl and 1-methylvinyl-carbonyl.

Examples of the pentafluorophenoxy compounds having an alkylcarbonylgroup include pentafluorophenyl acetate, pentafluorophenyl propionate,pentafluorophenyl butyrate, pentafluorophenyl trifluoroacetate,pentafluorophenyl pentafluoropropionate, pentafluorophenyl acrylate, andpentafluorophenyl methacrylate.

Examples of the alkoxycarbonyl groups having 2-12 carbon atoms includemethoxycarbonyl, ethoxycarbonyl, propoxycarbonyl, butoxycarbonyl,pentyloxycarbonyl, hexyloxycarbonyl, heptyloxycarbonyl,octyloxycarbonyl, nonyloxycarbonyl, decyloxycarbonyl, anddodecyloxycarbonyl. Further, branched alkoxycarbonyl groups such asisopropoxycarbonyl, tert-butoxycarbonyl, and 2-ethyl-hexyloxycarbonylcan be mentioned.

Furthermore, substituting groups in which at least one hydrogen atomcontained in the substituting group is replaced with a halogen atom oran aryl group having 6-18 carbon atoms can be mentioned. Examples of thelast substituting groups are alkoxycarbonyl groups such as1-chloroethoxycarbonyl, 2-chloroethoxycarbonyl,2,2,2-trofluoroethoxycarbonyl, 2,2,2-trichloroethoxycarbonyl, andbenzyloxycarbonyl.

Examples of the pentafluorophenoxy compounds having an alkoxycarbonylgroup include methyl pentafluorophenylcarbonate, ethylpentafluorophenylcarbonate, tert-butyl pentafluorophenylcarbonate,9-fluorenylmethyl pentafluorophenylcarbonate, and 2,2,2-trifluoroethylpentafluorophenylcarbonate.

An example of the aryloxycarbonyl group having 7-18 carbon atoms is o-,m-, or p-tolyloxycarbonyl group.

Examples of the pentafluorophenoxy compounds having an arylokycarbonylgroup include phenyl pentafluorophenylcarbonate and dipentafluorophenylcarbonate.

Examples of the alkanesulfonyl group having 1-12 carbon atoms includemethanesulfonyl, ethanesulfonyl, propanesulfonyl, butanesulfonyl,pentanesulfonyl, hexanesulfonyl, heptanesulfonyl, octanesulfonyl,nonanesulfonyl, decanesulfonyl, and dodecanesulfonyl. Further, branchedalkanesulfonyl groups such as 2-propanesufonyl can be mentioned.

Furthermore, substituting groups in which at least one hydrogen atomcontained in the substituting group is replaced with a halogen atom.Examples of the last substituting groups are trifluoromethanesulfonyland 2,2,2-trifluoroethanesulfonyl.

Examples of the pentafluorophenoxy compounds having an alkanesulfonylgroup include pentafluorophenyl methanesulfonate, pentafluorophenylethanesulfonate, pentafluorophenyl propanesulfonate, pentafluorophenyltrifluoromethanesulfonate, and pentafluorophenyl2,2,2-trifluoroethanesulfonate.

In the invention, one or more pentafluorphenoxy compounds are containedin the non-aqueous electrolytic solution. If the pentafluorophenoxycompound of the formula (I) is contained in the non-electrolyticsolution in an excessive amount, the battery performances may lower. Ifthe amount is too small, the expected improvement in the batteryperformance may not appear. Accordingly, the amount in the non-aqueouselectrolytic solution generally is 0.01 to 20 wt. %, preferably 0.05 to10 wt. %, more preferably 0.1 to 5 wt. %, from the viewpoint of increaseof the cycle performance.

Examples of the non-aqueous solvents employed in the invention arecyclic carbonates such as ethylene carbonate (EC), propylene carbonate(PC), butylene carbonate (BC), and vinylene carbonate (VC), lactonessuch as γ-butylolactone, linear carbonates such as dimethyl carbonate(DMC), methyl ethyl carbonate (MEC), and diethyl carbonate (DEC), etherssuch as tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane,1,2-dimethoxyethane, 1,2-diethoxyethane, and 1,2-dibutoxyethane,nitriles such as acetonitrile, aliphatic esters such as methylpropionate, methyl pivalate, butyl pivalate, and octyl pivalate, andamides such as dimethylformamide.

The non-aqueous solvents can be employed singly or in combination. Thereare no specific limitations with respect to the combination of thenon-aqueous solvents. Examples of the combinations include a combinationof a cyclic carbonate and a linear carbonate, a combination of a cycliccarbonate and a lactone, a combination of three cyclic carbonates and alinear carbonate, a combination of a cyclic carbonate and a aliphaticester, a combination of a cyclic carbonate, a lactone and an aliphaticester, and a combination of a lactone and an aliphatic ester. If thenon-aqueous solvent comprises a combination of a cyclic carbonate andother carbonate (e.g., linear carbonate), a ratio of the cycliccarbonate and other carbonate preferably is in the range of 5:95 to45:55.

Examples of the electrolytic salts contained in the electrolyticsolution of the invention include LiPF₆, LiBF₄, LiClO₄, LiN(SO₂CF₃)₂,LiN(SO₂C₂F₅)₂, LiC(SO₂CF₃)₃, LiPF₄(CF₃)₂, LiPF₃(C₂F₅)₃, LiPF₃(CF₃)₃,LiPF₃(iso-C₃F₇)₃, and LiPF₅(iso-C₃F₇). These electrolytes can beemployed singly or in combination. The electrolyte can be incorporatedinto the non-aqueous solvent generally in such an amount as to give anelectrolytic solution of 0.1 M to 3 M, preferably 0.5 M to 1.5 M.

The electrolytic solution can be prepared, for instance, by mixing theabove-mentioned non-aqueous solvents; dissolving the above-mentionedelectrolytic salt; in the mixture; and further dissolving at least onepentafluorophenoxy compound of the aforementioned formula (I) in theresulting mixture.

The non-aqueous electrolytic solution of the invention is employable formanufacturing a secondary battery, particularly a lithium secondarybattery.

There are no specific limitations on other constituting materials to beused for manufacturing the secondary battery. Various conventionallyemployed constitutional materials can be used.

For instance, the active material of positive electrode is a compoundmetal oxide comprising cobalt, nickel or manganese, and lithium. Theactive material of positive electrode can be used singly or incombination. Examples of the compound metal oxides include LiCoO₂,LiMn₂O₄, LiNiO₂, and LiCo_(1-x)Ni_(x)O₂ (0.01<x<1). These compounds canbe employed in an optional combination such as a combination of LiCoO₂and LiMn₂O₄, a combination of LiCoO₂ and LiNiO₂, and a combination ofLiMn₂O₄ and LiNiO₂.

The positive electrode can be manufactured by kneading theabove-mentioned active material of positive electrode, anelectro-conductive material such as acetylene black or carbon black, anda binder such as poly(tetrafluoroethylene) (PTFE), poly(vinylidenefluoride) (PVDF), styrene-butadiene copolymer (SBR),acrylonitrile-butadiene copolymer (NBR) or carboxymethylcellulose (CMC)to give a positive electrode composition; coating and pressing thepositive electrode composition on a collector such as aluminum foil or alath plate of stainless steel; and heating the pressed composition invacuo at a temperature of approximately 50 to 250° C. for approximately2 hours.

As the active material of negative electrode, lithium metal, lithiumalloy, carbonaceous material capable of absorbing and releasing lithium(e.g., thermally decomposed carbonaceous material, coke, graphites suchas artificial graphite and natural graphite, fired organic polymer, andcarbon fiber), or a compound tin oxide is employable. It is preferred toemploy carbonaceous materials having a graphite crystal structure inwhich the lattice distance of lattice surface (002), namely, d₀₀₂, is inthe range of 0.335 to 0.340 nm (nanometer). The active materials ofnegative electrode can be employed singly or in combination. A powderymaterial such as the carbonaceous material is preferably used incombination with a binder such as ethylene propylene diene terpolymer(EPDM), polytetrafluoroethylene (PTFE), poly-(vinylidene fluoride)(PVDF), styrene-butadiene copolymer (SBR), acrylonitrile-butadienecopolymer (NBR), or carboxymethylcellulose (CMC). There are nolimitations with respect to the preparing method of the negativeelectrode. The negative electrode can be prepared by a method similar tothat for the preparation of the positive electrode.

There are no specific limitations with respect to the structure of thenon-aqueous lithium secondary battery of the invention. For instance,the non-aqueous secondary battery can be a battery of coin typecomprising a positive electrode, a negative electrode, and single orplural separators, or a cylindrical or prismatic battery comprising apositive electrode, a negative electrode, and a separator roll.

The separator can be a known material such as microporous polyolefinfilm, woven cloth, or non-woven cloth.

The present invention is further described by the following examples andcomparison examples.

EXAMPLE 1

1) Preparation of Non-aqueous Electrolytic Solution

In a non-aqueous solvent of PC:DMC (=1:2, volume ratio) was dissolvedLiPF₆ (electrolytic salt) to give a non-aqueous electrolytic solution of1M concentration. To the non-aqueous electrolytic solution was furtheradded 0.5 wt. % of pentafluorophenyl methanesulfonate.

2) Manufacture Lithium Secondary Battery and Measurement of its BatteryCharacteristics

LiCoO₂ (positive electrode active material, 90 wt. %), acetylene black(electro-conductive material, 5 wt. %), and poly(vinylidene fluoride)(binder, 5 wt. %) were mixed. To the resulting mixture was further added1-methyl-2-pyrrolidone. Thus produced mixture was coated on aluminumfoil, dried, pressed, and heated to give a positive electrode.

Artificial graphite (negative electrode active material, 90 wt. %) andpoly(vinylidene fluoride) (binder, 10 wt. %) were mixed. To theresulting mixture was further added 1-methyl-2-pyrrolidone. Thusproduced mixture was coated on copper foil, dried, pressed, and heatedto give a negative electrode.

The positive and negative electrodes, a microporous polypropylene filmseparator, and the above-mentioned non-aqueous electrolytic solutionwere employed to give a coin-type battery (diameter: 20 mm, thickness:3.2 mm).

The coin-type battery was charged at room temperature (20° C.) with aconstant electric current (1.1 mA) to reach 4.2 V for 5 hours.Subsequently, the battery was discharged to give a constant electriccurrent (1.1 mA) to give a terminal voltage of 2.7 V. Thecharging-discharging cycle test was repeated.

The initial discharge capacity was almost the same as the capacitymeasured in a battery using an 1M LiPF₆ and EC/DEC (3/7, volume ratio)solvent mixture (containing no additive) [see Comparison Example 2].

After the 50 cycle charging-discharging procedure, the retention ofdischarge capacity was 88.8% of the initial discharge capacity (100% ).

The manufacturing conditions and the battery performances are shown inTable 1.

EXAMPLE 2

The procedures of Example 1 for preparing a non-aqueous electrolyticsolution were repeated except for employing 1 wt. % of pentafluorophenylmethanesulfonate. Then, a coin-type battery was manufactured byemploying the resulting non-aqueous electrolytic solution.

The 50 cycle charging-discharging test was carried out, and it wasobserved that the retention of discharge capacity was 91.9%.

The manufacturing conditions and the battery performances are shown inTable 1.

EXAMPLE 3

The procedures of Example 1 for preparing a non-aqueous electrolyticsolution were repeated except for employing 2 wt. % of pentafluorophenylmethanesulfonate. Then, a coin-type battery was manufactured byemploying the resulting non-aqueous electrolytic solution.

The 50 cycle charging-discharging test was carried out, and it wasobserved that the retention of discharge capacity was 90.3%.

The manufacturing conditions and the battery performances are shown inTable 1.

EXAMPLE 4

The procedures of Example 1 for preparing a non-aqueous electrolyticsolution were repeated except for employing 1 wt. % of pentafluorophenylacetate. Then, a coin-type battery was manufactured by employing theresulting non-aqueous electrolytic solution.

The 50 cycle charging-discharging test was carried out, and it wasobserved that the retention of discharge capacity was 90.6%.

The manufacturing conditions and the battery performances are shown inTable 1.

EXAMPLE 5

The procedures of Example 1 for preparing a non-aqueous electrolyticsolution were repeated except for employing 1 wt. % of methylpentafluorophenylcarbonate. Then, a coin-type battery was manufacturedby employing the resulting non-aqueous electrolytic solution.

The 50 cycle charging-discharging test was carried out, and it wasobserved that the retention of discharge capacity was 89.7%.

The manufacturing conditions and the battery performances are shown inTable 1.

EXAMPLE 6

The procedures of Example 1 for preparing a non-aqueous electrolyticsolution were repeated except for employing a non-aqueous solvent ofEC/MEC (3/7, volume ratio) and 1 wt. % of pentafluorophenylmethanesulfonate. Then, a coin-type battery was manufactured byemploying the resulting non-aqueous electrolytic solution.

The 50 cycle charging-discharging test was carried out, and it wasobserved that the retention of discharge capacity was 92.2%.

The manufacturing conditions and the battery performances are shown inTable 1.

EXAMPLE 7

The procedures of Example 1 for preparing a non-aqueous electrolyticsolution were repeated except for employing a non-aqueous solvent ofEC/MEC (3/7, volume ratio) and 1 wt. % of pentafluorophenyl acetate.Then, a coin-type battery was manufactured by employing the resultingnon-aqueous electrolytic solution.

The 50 cycle charging-discharging test was carried out, and it wasobserved that the retention of discharge capacity was 91.7%.

The manufacturing conditions and the battery performances are shown inTable 1.

EXAMPLE 8

The procedures of Example 1 for preparing a non-aqueous electrolyticsolution were repeated except for employing a non-aqueous solvent ofEC/MEC (3/7, volume ratio) and 1 wt. % of methylpentafluorophenylcarbonate. Then, a coin-type battery was manufacturedby employing the resulting non-aqueous electrolytic solution.

The 50 cycle charging-discharging test was carried out, and it wasobserved that the retention of discharge capacity was 91.4%.

The manufacturing conditions and the battery performances are shown inTable 1.

EXAMPLE 9

The procedures of Example 1 for preparing a non-aqueous electrolyticsolution were repeated except for employing a non-aqueous solvent ofEC/DEC (1/2, volume ratio) and 1 wt. % of pentafluorophenylmethanesulfonate. Then, a coin-type battery was manufactured byemploying the resulting non-aqueous electrolytic solution.

The 50 cycle charging-discharging test was carried out, and it wasobserved that the retention of discharge capacity was 92.3%.

The manufacturing conditions and the battery performances are shown inTable 1.

EXAMPLE 10

The procedures of Example 1 for preparing a non-aqueous electrolyticsolution were repeated except for employing LiMn₂O₄ in place of LiCoO₂and 1 wt. % of pentafluorophenyl methanesulfonate. Then, a coin-typebattery was manufactured by employing the resulting non-aqueouselectrolytic solution.

The 50 cycle charging-discharging test was carried out, and it wasobserved that the retention of discharge capacity was 88.1%.

The manufacturing conditions and the battery performances are shown inTable 1.

EXAMPLE 11

The procedures of Example 1 for preparing a non-aqueous electrolyticsolution were repeated except for employing natural graphite in place ofartificial graphite and 1 wt. % of pentafluorophenyl methanesulfonate.Then, a coin-type battery was manufactured by employing the resultingnon-aqueous electrolytic solution.

The 50 cycle charging-discharging test was carried out, and it wasobserved that the retention of discharge capacity was 90.5%.

The manufacturing conditions and the battery performances are shown inTable 1.

COMPARISON EXAMPLE 1

In a non-aqueous solvent of PC:DMC (=1:2, volume ratio) was dissolvedLiPF₆ to give a non-aqueous electrolytic solution of 1M concentration.To the non-aqueous electrolytic solution was added no pentafluorophenoxycompound.

Then, a coin-type battery, was manufactured by employing the resultingnon-aqueous electrolytic solution.

In the battery performance test, no charging-discharging was observed.

COMPARISON EXAMPLE 2

In a non-aqueous solvent of EC:DEC (=3:7, volume ratio) was dissolvedLiPF₆ to give a non-aqueous electrolytic solution of 1M concentration.To the non-aqueous electrolytic solution was added no pentafluorophenoxycompound. Then, a coin-type battery was manufactured by employing theresulting non-aqueous electrolytic solution.

The 50 cycle charging-discharging test was carried out, and it wasobserved that the retention of discharge capacity was 82.1%.

The manufacturing conditions and the battery performances are shown inTable 1.

TABLE 1 Initial ca- Electrode Additive pac- 50% cycle Exam- Posi.(amount: Electrolytic ity retention ple Nega. wt. %) solution (r.v.) (%)1 LiCoO₂ Pentafluoro- 1M LiPF₆ 1.00 88.8 Art. phenyl methane- PC/DMC =sulfonate (0.5) 1/2 2 LiCoO₂ Pentafluoro- 1M LiPF₆ 1.01 91.9 Art. phenylmethane- PC/DMC = sulfonate (1) 1/2 3 LiCoO₂ Pentafluoro- 1M LiPF₆ 1.0290.3 Art. phenyl methane- PC/DMC = sulfonate (2) 1/2 4 LiCoO₂Pentafluoro- 1M LiPF₆ 1.01 90.6 Art. phenyl acetate PC/DMC = (1) 1/2 5LiCoO₂ Methyl Penta- 1M LiPF₆ 1.00 89.7 Art. fluorophenyl- PC/DMC =carbonate (1) 1/2 6 LiCoO₂ Pentafluoro- 1M LiPF₆ 1.02 92.2 Art. phenylmethane- EC/MEC = sulfonate (1) 3/7 7 LiCoO₂ Pentafluoro- 1M LiPF₆ 1.0291.7 Art. phenyl acetate EC/MEC = (1) 3/7 8 LiCoO₂ Mechyl Penta- 1MLiPF₆ 1.02 91.4 Art. fluorophenyl- EC/MEC = carbonate (1) 3/7 9 LiCoO₂Pentafluoro- 1M LiPF₆ 1.01 92.3 Art. phenyl methane- EC/DEC = sulfonate(1) 1/2 10  LiMn₂O₄ Pentafluoro- 1M LiPF₆ 0.85 88.1 Art. phenyl methane-PC/DMC = sulfonate (1) 1/2 11  LiCoO₂ Pentafluoro- 1M LiPF₆ 0.99 90.5Nat. phenyl methane- PC/DMC = sulfonate (1) 1/2 Con. LiCoO₂ None 1MLIPF₆ — Failure 1 Art. PC/DMC = 1/2 Con. LiCoO₂ None 1M LiPF₆ 1 82.1 2Art. EC/DEC = 3/7 Remarks: PC: Propylene carbonate DMC: Dimethylcarbonate EC: Ethylene carbonate MEC: Methyl ethyl carbonate

INDUSTRIAL UTILITY

The present invention provides a lithium secondary battery havingexcellent battery performances in the cycle performance, electriccapacity, and storage performance.

1. A non-aqueous electrolytic solution comprising an electrolytic saltin a non-aqueous solvent, which further contains a pentafluorophenoxycompound having the formula (I):

in which R represents a substituting group selected from the groupconsisting of an alkylcarbonyl group having 2-12 carbon atoms, analkoxycarbonyl group having 2-12 carbon atoms, an aryloxycarbonyl grouphaving 7-18 carbon atoms, and an alkanesulfonyl group having 1-12 carbonatoms, provided that at least one hydrogen atom contained in thesubstituting group can be replaced with a halogen atom or an aryl grouphaving 6-18 carbon atoms.
 2. The non-aqueous electrolytic solutiondefined in claim 1, in which the pentafluorophenoxy compound iscontained in the electrolytic solution in an amount of 0.01 to 20 wt. %.3. The non-aqueous electrolytic solution defined in claim 1, in whichthe pentafluorophenoxy compound is contained in the electrolyticsolution in an amount of 0.05 to 10 wt. %.
 4. The non-aqueouselectrolytic solution defined in claim 1, in which thepentafluorophenoxy compound is contained in the electrolytic solution inan amount of 0.1 to 5 wt. %.
 5. The non-aqueous electrolytic solutiondefined in claim 1, in which the pentafluorophenoxy compound is at leastone compound selected from the group consisting of pentafluorophenylacetate, pentafluorophenyl propionate, pentafluorophenyl butyrate,pentafluorophenyl trifluoroacetate, pentafluorophenylpentafluoropropionate, pentafluorophenyl acrylate, and pentafluorophenylmethacrylate.
 6. The non-aqueous electrolytic solution defined in claim1, in which the pentafluorophenoxy compound is at least one compoundselected from the group consisting of methyl pentafluorophenylcarbonate,ethyl pentafluorophenylcarbonate, tert-butyl pentafluorophenylcarbonate,9-fluorenylmethyl pentafluorophenylcarbonate, and 2,2,2-trifluoroethylpentafluorophenylcarbonate.
 7. The non-aqueous electrolytic solutiondefined in claim 1, in which the pentafluorophenoxy compound is at leastone compound selected from the group consisting of phenylpentafluorophenylcarbonate and dipentafluorophenyl carbonate.
 8. Thenon-aqueous electrolytic solution defined in claim 1, in which thepentafluorophenoxy compound is at least one compound selected from thegroup consisting of pentafluorophenyl methanesulfonate,pentafluorophenyl ethanesulfonate, pentafluorophenyl propanesulfonate,pentafluorophenyl trifluoromethanesulfoanate, and pentafluorophenyl2,2,2-trifluoroethanesulfonate.
 9. The non-aqueous electrolytic solutiondefined in claim 1, in which the pentafluorophenoxy compound is at leastone compound selected from the group consisting of pentafluorophenylmethanesulfonate, pentafluorophenyl acetate, and methylpentafluorophenylcarbonate.
 10. The non-aqueous electrolytic solutiondefined in claim 1, in which the non-aqueous solvent is a mixture of acyclic carbonate and a linear carbonate.
 11. The non-aqueouselectrolytic solution defined in claim 10, in which a volume ratiobetween the cyclic carbonate and the linear carbonate is in the range of5:95 to 45:55.
 12. The non-aqueous electrolytic solution defined inclaim 1, in which the electrolytic salt is at least one salt selectedfrom the group consisting of LiPF₆, LiBF₄, LiClO₄, LiN(SO₂CF₃)₂,LiN(SO₂C₂F₅)₂, LiC(SO₂CF₃)₃, LiPF₄(CF₃)₂, LiPF₃(C₂F₅)₃, LiPF₃(CF₃)₃,LiPF₃(iso-C₃F₇)₃, and LiPF₅(iso-C₃F₇).
 13. A lithium secondary batterywhich comprises a positive electrode, a negative electrode, and anon-aqueous electrolytic solution comprising an electrolytic salt in anon-aqueous solvent, in which the non-aqueous electrolytic solutionfurther contains a pentafluorophenoxy compound having the formula (I):

in which R represents a substituting group selected from the groupconsisting of an alkylcarbonyl group having 2-12 carbon atoms, analkoxycarbonyl group having 2-12 carbon atoms, an aryloxycarbonyl grouphaving 7-18 carbon atoms, and an alkanesulfonyl group having 1-12 carbonatoms, provided that at least one hydrogen atom contained in thesubstituting group can be replaced with a halogen atom or an aryl grouphaving 6-18 carbon atoms.
 14. The lithium secondary battery of claim 13,in which the pentafluorophenoxy compound is contained in theelectrolytic solution in an amount of 0.01 to 20 wt. %.
 15. The lithiumsecondary battery of claim 13, in which the pentafluorophenoxy compoundis contained in the electrolytic solution in an amount of 0.1 to 5 wt.%.
 16. The lithium secondary battery of claim 13, in which the positiveelectrode comprises at least one active material selected from the groupconsisting of LiCoO₂, LiMn₂O₄, LiNiO₂, and LiCo_(1-x)Ni_(x)O₂ under thecondition of 0.01<x<1.
 17. The lithium secondary battery of claim 13, inwhich the negative electrode comprises artificial graphite or naturalgraphite as active material.